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In order to understand the influence of source rock types and maturity on hydrocarbon gases carbon isotope change more objectively, a closed-system pyrolysis experiment was carried out on six samples from 250 to 550°C. The values of δ 13 C 1 , δ 13 C 2 , and δ 13 C 3  were ranged from −73.3 to −29.8%, from −36.3 to −9.5%, and from −38.5 to −12.4%, respectively. The range of δ 13 C 1  was the largest, reaching a top value of 43.5%. The results showed that the temperature has an effect on the carbon isotope value of pyrolysis gas. With the increase of the degree of thermal evolution, the carbon isotope value of methane in all samples, except for huangxian gangue, had a change trend from heavy to light firstly, then got heavier. In addition, the carbon isotope values of methane, ethane and propane had the features of δ 13 C 1  &amp;lt; δ 13 C 2  &amp;lt; δ 13 C 3  when the temperatures were under 550°C, which were made up of a series of positive carbon isotopes. However, when the temperature increased above 550°C, there was an inversion of the simulated gas carbon isotope values in Huangxian coal gangue, Minqin oil shale and Huaan carbonaceous shale, i.e., δ 13 C 2  &amp;gt; δ 13 C 3  and δ 13 C 2  &amp;gt; δ 1 3C 1 . It indicates that the secondary cracking has occurred at high maturity or over maturity stage.

Inversion Characteristics of Hydrocarbon Gases Carbon Isotopes Varying With Temperature and Implications for Shale Exploration